1. Field of the Invention
This invention relates to helmet mounted displays that generate images which constitute all that the observer sees or which are combined with the observer's normal view of the surrounding environment.
2. Description of the Related Art
Helmet mounted stereo displays have been around at least since McCullum described a wireless system using two cathode ray tubes (CRTs) in 1943 (U.S. Pat. No. 2388170). They have been used in avionics, tanks, as sights for soldiers and helicopter pilots, in surgery, at workstations, for robotics and in entertainment devices.
U.S. Pat. Nos. 4,982,278, 4,884,137, 4,970,589, 4,874,214, 4,761,056, 4,398,799, 4,465,347, 3,614,314, 305,919, 2,955,156, 4,859,030, 4,269,476, 4,322,135, 4,761,056, 4,969,724, 4,902,116, 4,468,101, 4,775,217, 4,968,123, 4,961,626, 4,969,714, 4,897,715, 4,028,725, 4,571,628, 4,982,278, 4,933,755, 4,952,024, 4,805,988, 4,385,803, 4,853,764 and 4,874,235 all have some elements of their design in common with the present invention. Most of these designs have complex refractive optical trains to relay the image from a CRT or liquid crystal display (LCD) to the eye. They typically utilize a combination of lenses, mirrors, and holographic optics; a few include fiber optics in some form. Some are monochromatic and some are polychromatic; some, binocular; and a few are biocular (both eyes see the same image).
Foreign patents that are related include German nos. 3532730, 3628458, 224691, and 3532730; French no. 2517916; and Japanese no. 63-82192, 63-177689, 59-117889, 59-219092, 62-272698, and 1-61723WO 84/01680. Most of these are heads up displays (HUD) where the projected image is made to overlay a see through view of the real world, but some are for non-see through wide angle virtual reality projection such as the contoured fiber optic design of Webster, U.S. Pat. No. 4,874,235. In Webster's design two contoured fiber optic bundles are made concave to extend the field of view (FOV) and also greatly distort the input image which is viewed with strong contact lenses, and no see through light is allowed. A coherent image conduit of fibers is in effect an image relay system that can also change the size and shape of an image to control the field of view or concentrate the image light from a large display via a tapered section of fibers. It is also a well known practice to use flexible coherent bundles as image conduit over a meter or more and, furthermore, to divide the bundle into smaller bundles with tapers attached to increase the total number of pixels displayed (SBWP) by combining multiple remote inputs to make one high-resolution output. This practice is not a part of the present invention, and it is extremely expensive to implement.
The popular LEEP lens design of Teitel U.S. Pat. No. 4,982,278, could also not be made to work in a see through configuration because the eye relief (the distance between the eye and the first optical element) is too short to insert a folding optic of any sort, which is necessary for most wide FOV see through or heads up display (HUD) designs.
In the present invention., holographic optical elements (HOEs) have been used to make the fold mirror and the collimator-combiner. Because of this it is useful to know a little about the history of HOEs in spite of the fact that they are not necessary to the workings of this invention, as they may be replaced by conventional dielectric mirrors made commercially in a vacuum chamber.
Historically HOEs have been used as combiners and or collimators in helmet mounted displays (HMDs) at least since Don Close and Andrejs Graube of Hughes aircraft company (Air Force contract no. F33615-73-C-4110), and others at Hughes Aircraft put them on visors in the early 1970s. One fabrication method practiced then was film transfer, a process whereby the hologram was formed in gelatin and processed on a stable flat glass substrate and subsequently loosened, lifted off, and transferred to the cylindrical plastic visor where it was then laminated with a matching visor. The method required considerable skill and patience and some unusual exposure optics to compensate for the cylindrical surface.
An alternative to their method involves subbing (preparing the surface for an aqueous solution) the spherical or cylindrical plastic so that it will take a sensitized gelatin coating and then holding the coated plastic rigidly to obtain the required stability for a two beam exposure to laser light. This method requires developing ways to sub and coat with good optical quality but results in well-corrected optics with relatively low dispersion and flare light compared to flat plates. This method probably has the greatest potential for perfect imaging over the largest field of view but will always leave some flare light from fringes at the substrate and cover interface and, if made for more than one color, will have considerable chromatic aberration. It is the basis of U.S. Pat. No. 4,874,214 belonging to Thompson CSF of France. The present invention will not have any flare light nor any chromatic aberration because, unlike the prior art, the reflection hologram is made to conform to the shape of the plastic sustrate, which is spherical.
When flare light is not a problem for the user and optical bandwidths on playback are less than 15 nm then the HOE on a flat glass or plastic substrate may be used with good imaging quality over a field of view (FOV) of about 20 degrees. These elements have been made with good success by using aspheric optics, decentered or tilted optics and/or computer generated diffractive optics (CGH) to generate the optimum interfering wavefronts. More recently they have been made using simple spherical waves to form intermediate HOEs that will generate the optimum corrected wavefronts when reconstructed in a different geometry or at a different wavelength from the one in which they were originally constructed. This recent method, called a recursive design technique, was developed by Amitai and Friesem and is relatively easy to implement. Variations of this approach were introduced earlier by Ono in the design of holographic scanners, collimators and transform optics and were applied to a biocular HUD design for the U.S. Air Force (contract F33615-86-C-3618) by Richard D. Rallison in 1986. The major advantage with this method is the ease of manufacturing on flat plates. The disadvantages are the limited field of view and the necessity of using only one wavelength with a narrow spectral bandwidth, typically about 15 nanometers. The present invention has no such restrictions on either bandwidth or FOV.
A popular design in recent years is the result of the requirement to make FOVs increasingly larger and to make scenes more natural in color or simply more colorful for better or faster interpretation of displayed information. This design goal may be met by employing an element referred to as a spherical conformal holographic reflector, which has no dispersive qualities and a controllable spectral bandwidth. It has been made and used by many investigators for HUDs or HMDs, including Wood, Margarinos, Tedesco and others, mainly for aircraft and eye protection and is in some ways the simplest holographic element to produce. Fabrication requires the coating of spherical glass or plastic substrates, such as an ophthalmic lens blank, with a holographic quality film but requires only a single beam of low coherence to expose. The object wave is derived from a portion of the reference wave reaching the conformal mercury or simply the film-air interface. This is the preferred method of making a collimator-combiner for the present invention or, if a virtual reality display (VRD) is being made, then the same blank may be coated with aluminum in a vacuum chamber.
A similar technology is used on the ophthalmic (lens blank) substrates popular in several folded catadioptric (CAT) HMD designs. At Honeywell the substrates have been coated by vapor deposition of dielectrics in a vacuum. In addition to folding the optical path, some of their designs have a tilt angle other than 45 degrees between the fold mirror and the collimator to improve image light efficiency. As long as the tilt angle in these systems can remain small, simple inexpensive spherical optics can be used even for a FOV of 50 degrees. Typical tilt, or off-axis angles, range from 6 to 12 degrees with cost and complexity of associated corrective refractive optics going up geometrically as the angle increases. Designs with zero tilt angle do not let the image light that was folded onto the collimator pass through the fold mirror to the eye without a loss in intensity. Much of the collimated light impinges on the fold mirror at about the same angle that it was originally folded toward the collimator, which causes it to be reflected back to the image source, rather than pass through to the eye. The introduction of the tilt allows the light from the collimator to impinge on the fold mirror at an angle at which it does not reflect efficiently. This tilt generates aberrations that are difficult to correct.